The disclosure relates to a novel lubricating oil for the compressor of a refrigeration system. Typically a compression type refrigeration circuit comprises a compressor, a condenser, expansion device(s) and an evaporator. The refrigerant and compressor lubricant should most typically circulate throughout the system without phase separation. Potential consequences of phase separation may include inadequate lubrication of the compressor and reduced heat exchange efficiency. Lubricating oils for refrigeration systems most preferentially also demonstrate a high viscosity index, thus ensuring viscosity retention and hence adequate lubrication across the system's temperature range. Appropriate pressure, viscosity and temperature dynamics are required. Furthermore, an appropriate level of chemical, thermal and hydrolytic stability is required of the lubricant composition in contact with the refrigerant and system componentry across the operating temperature range. Most commonly a low level of hygroscopicity is also considered to be a requirement of the lubricant to limit potential of ice blockage of the expansion device and chemical instability due to potential component corrosion resulting from the formation of acidic species in the presence of excessive moisture levels.
Refrigerants composed partly or in entirety of fluoro-olefin (“fluoroalkene”), especially hydrofluoro-olefin type refrigerant, are under active development as a more environmentally friendly refrigerant for use in automotive and stationary air-conditioning and refrigeration systems. The novelty of this refrigerant type requires a refrigeration compressor lubricant affording properties specific to this refrigerant type, such properties specifically include appropriate refrigerant compatibility, and elevated levels of chemical, thermal and hydrolytic stability due to potential reactivity of the hydrofluoro-olefin refrigerant compared to traditional hydrofluorocarbon refrigerants. With an increasing automotive industry focus on utilizing hybrid and electric air-conditioning compressors to realize vehicle fuel savings, there is a further requirement that the lubricant technology defined for belt-driven compressors should be equally applicable for use in electrically driven compressors, where the electrical properties of the lubricant also require specification. Lubricant compositions considered suitable for use with the new hydrofluoro-olefin refrigerants include those with higher polarity and hence greater miscibility with the refrigerant, such as polyol esters (POEs) and polyalkylene glycols (also known as, and referred to herein as polyoxyalkylene glycols, PAGs) whereas less polar lubricant types such as mineral oils and alkylbenzenes lack the polarity required and hence demonstrate immiscibility with hydrofluoro-olefins.
Honeywell's US 2007/0069175 discloses mixtures of various fluoroalkene refrigerants with various lubricants which may include organic lubricants of the PAG and POE type. This document discloses a variety of lubricant types but does not demonstrate particular suitability of one lubricant type or denominations thereof, compared to another. US 2010/0205980 discloses that PAGs of a particular structure may serve as basefluids for use with a fluoro-olefin refrigerant.
Within the refrigeration industry, there is a strong antipathy to the use of mixed, or hybrid, lubricants. The use of mixtures of different lubricant base fluids can in some cases lead to serious disadvantages, and the predominant practice is to use a single base fluid as lubricant. Manufacturers of refrigeration equipment go to some lengths to ensure that during maintenance of their equipment, the correct base fluid is used, fearing that the use of a different base fluid from that used originally could cause damage.
US 2011/019-184 references MACS Service Report, March 2008, p. 1-8 by Weissler, saying: “Denso has shown that 1% of PAG oils (such as its ND8) in an electric motor-drive compressor system lowers resistance to about 1 megaohm, well below the over-10 megaohm provided by the usage of 100% of its ND11 oil, a POE.” The Weissler report itself states: “When SAE J2788, the current standard for recovery/recycle/recharging equipment was written, we recognized the issue of oil cross-contamination in the electric-motor compressors of some hybrids. It obviously was important to minimize getting PAG into the POE oil . . . and it was for this reason that SAE J2788 sets a limit of 1%, well below some once-heard estimates that up to 8% could be tolerated. It turns out that both numbers were off the mark. The limit, along with a new, severe test, is likely to be going down to just 0.1% (100 parts per million of PAG in the POE oil), to leave a good margin for safety.” This typifies certain prejudice in the art against the use of PAG and POE mixtures as base fluids for refrigeration systems.
However, disclosed herein is the use of POE/PAG mixtures of particular composition with fluoro-olefin, especially HFO, refrigerants that demonstrate a surprising effect which could not have been predicted. Specifically, the addition of a minor amount of a PAG to a POE base fluid has a hygroscopic or desiccant effect, which in turn reduces or eliminates the need to add acidity regulators to the POE base fluid when used with such refrigerants. This makes such mixtures particularly suitable for use with such refrigerants.